A picture here would really help there are two standard heaters for 1950 neither of which is really square?
https://www.flickr.com/photos/advance-design_parts_co/8129448404/in/album-72157631868452754/
If you have this one which is correct for 47-48 deluxe and super deluxe plaques exist, you see them with and without the add-on defroster.
https://www.flickr.com/photos/advance-design_parts_co/49141664128/in/album-72157631868452754/
This heater could have lingered for years on a dealer shelf or been retrofitted from another truck. If it is not one of these three you are in Canada or have an aftermarket.